1. Field of the Invention
The present invention relates to a regulated DC voltage power supply that provides a DC load with a regulated DC voltage, and particularly to a circuit configuration of the regulated DC voltage power supply.
2. Description of Related Art
Various types of DC converters, such as forward type, flyback type, chopper type, etc., are used as a regulated DC voltage power supply for supplying a regulated DC voltage to diverse DC loads like electronic apparatuses.
FIGS. 1A-1C show forward type, flyback type, and down-voltage chopper type regulated DC voltage power supplies, respectively. In these figures, the reference numeral 10 designates a power circuit. The power circuit 10 of the forward type comprises a pair of input terminals 11, a MOSFET 12, a transformer 13, diodes 14 and 15, a filter 16, and a pair of output terminals 19. Here, the input terminals 11 receive a DC input voltage Vi supplied from a DC power supply not shown in this figure, and the MOSFET 12 chops the input DC voltage Vi. The transformer 13 comprises a primary winding 13a and a secondary winding 13b, and transforms the chopped DC voltage. The anode of the diode 14 is connected to a first terminal of the secondary winding 13b, the anode of the diode 15 is connected to a second terminal thereof, and the cathodes of the diodes 14 and 15 are connected in common to an input terminal 14a of the filter 16. The filter 16 comprises a reactor 16a connected between the terminal 14a and one of the output terminals 19, and a capacitor 16b connected between the terminals 19.
The power circuit 10 of the flyback type as shown in FIG. 1B differs from the power circuit 10 of the forward type of FIG. 1A in the following: It comprises a diode 14, and a filter 17 which includes a transformer 18 and a capacitor 16b, but does not comprise the transformer 13 and the diode 15. The transformer 18 comprises a primary winding 18a and a secondary winding 18b, and transforms the chopped DC voltage to be supplied to the diode 14. In addition, the transformer 18 functions as a reactor for smoothing. The output of the transformer 18 is rectified by the diode 14, and the rectified voltage is smoothed by the secondary winding 18b and the capacitor 16b.
The power circuit 10 of the down-voltage chopper type as shown in FIG. 1C differs from the power circuit 10 of the forward type of FIG. 1A in the following: It comprises a filter 16 and a diode 15, but does not comprise the transformer 13 and the diode 14. The filter 16 comprises a reactor 16a and a capacitor 16b, a first terminal of the reactor being connected to the cathode of the diode 15, and a second terminal thereof being connected to the capacitor 16b. The cathode of the diode 15 is further connected to the MOSFET 12 and the anode thereof is connected to the common line.
The reference numeral 100 designates a control circuit which delivers to the MOSFET 12 a switching signal that turns on and off the MOSFET 12 with a duty ratio specified in accordance with the DC output voltage Vo. To accomplish this, the control circuit 100 comprises a voltage divider 101, a detecting line 102, a control IC 103, and a drive circuit 105. Here, the voltage divider 101 comprises resistors 101a and 10lb serially connected across the output terminals 19, and functions as a detector detecting the DC output voltage Vo. The detecting line 102 connects the connecting point 101c of the resistors 101a and 10lb to the input terminal 103a of the control IC 103. In this case, the voltage at the input terminal is 1/k of the DC output voltage Vo, and corresponds to a load quantity required by a load. The control IC 103 produces a switching signal 104 whose duty ratio is determined in accordance with the voltage Vo/k at the input terminal 103a. More specifically, when the voltage Vo/k is equal to a predetermined reference voltage Vs, the duty ratio of the switching signal 104 is unchanged, when the voltage Vo/k is less than the reference voltage Vs, the duty ratio is increased, and when the voltage Vo/k is greater than the reference voltage Vs, the duty ratio is decreased. The drive circuit 105 amplifies the power of the switching signal 104 to generate a drive signal 106, and supplies the drive signal 106 to the gate 12a of the MOSFET 12.
Thus, the conventional regulated DC voltage power supply, irrespective of its type, switches the MOSFET 12 in accordance with the duty ratio of the driving signal 106 supplied from the control circuit 100. The MOSFET 12 is made conductive for a time Ton and nonconductive for a following time Toff, repeating the on-off operation at a cycle of Tc=Ton+Toff. Thus, the MOSFET 12 outputs the chopped voltage of the DC input voltage Vi which is fed to the filter 16 or 17 to be smoothed directly or via the transformer. The DC voltage which is smoothed to remove pulsating components is outputted from the output terminals. 19. Thus, the regulated DC output voltage Vo is produced whose voltage equals Vs.times.k, and which contains little ripples.
The relationship between the DC output voltage Vo and the DC input voltage Vi is expressed as the following equation (1) in the case of the forward type regulated DC voltage power supply. EQU Vo.apprxeq.Vi.times.r.times.(Ton/Tc) (1)
where r is the turns ratio of the transformer 13, that is, (the number of turns of the secondary winding 13b)/(the number of turns of the primary winding 13a), arid (Ton/Tc) is the duty ratio.
When the regulated DC voltage power supply is mounted on an electronic apparatus, it is usually packaged on a printed board. As the performance of electronic apparatuses become higher, the increasing number of small capacity regulated DC voltage power supplies whose capacity is several to several tens of watts are required to be incorporated into individual electric circuits. To meet such requirements, the reduction in size of DC voltage regulated power supplies has been satisfied by employing the following techniques. First, semiconductor devices such as the semiconductor switch (MOSFET) 12, the diodes 14 and 15 the control IC 103, and the drive circuit 105 are incorporated on a single silicon chip. Second, the operation frequency f (f=1/Tc) for switching the semiconductor switch (MOSFET12) is set at a high frequency such as several hundred kilohertz. This is not only because the reactances of the reactor 16a and the transformer 18 used in the filters 16 and 17, and the capacitance of the capacitor 16b can be reduced in inverse proportion to the operation frequency f to achieve identical smoothing effects, but also because the cross-sectional areas of the cores of the transformers 13 and 18 can be reduced in inverse proportion to the operation frequency f when the flux density is maintained constant.
According to the conventional technique, it is possible to supply regulated DC voltages to loads using products considerably reduced in size.
Recently, however, electronic devices have been sharply reducing their size and weight owing to miniaturization of semiconductor chips such as LSI components employing fine processing, and high-density packaging techniques. In such situations, the following problems have arisen in connection with conventional DC voltage regulated power supplies.
(1) Operation frequencies higher than 1 MHz will sharply increase losses in the cores of the transformers, thereby making it impossible to maintain the flux density constant. As a result, the reduction in size by employing a high operation frequency reaches its limit. Furthermore, in the case of reactors, an adverse effect due to stray capacity is added.
(2) The loss of the semiconductor switch will increase with operation frequency. For example, a loss analysis of a flyback type regulated DC voltage power supply which was operated at 1 MHz provided the following results: 35% of the total loss arose from the semiconductor switch; 20% thereof arose from the transformer; and the remaining 45% arose from the other components. This shows that the semiconductor switch presents the greatest single loss, and this hinders the operation frequency from being increased further.
(3) Although the size and number of the components has been reduced in the conventional regulated DC voltage power supply, its individual components are still mounted on a printed board. As a result, the miniaturization of the power supply itself reaches its limit owing to the size of the printed board. Furthermore, since active parts and passive parts are connected via conductor layers on a printed board, the operation of the power supply may be adversely affected by the stray inductance of the conductor layers when the operation frequency exceeds 1 MHz. As a result, the variation in performance of the power supply and erroneous operation due to external noise induced in the conductor layers will increase, thereby reducing the reliability of the power supply. Thus, the miniaturization of the regulated DC voltage power supply by increasing the operation frequency has reached its limit.